Air bubble-initiated biofabrication of freestanding, semi-permeable biopolymer membranes in PDMS microfluidics

• Semi-permeable biopolymer membranes are assembled in situ in microfluidic channels.
• Chitosan and alginate form polyelectrolyte complex membrane via ionic interactions.
• Membranes are assembled in situ via sacrificial microairbubble scaffold.
• Membrane porosity enables small molecule penetration, not antibody transport.

Membrane functionality in microfluidics is critical for sample separation, concentration, compartmentalization, filtration, pumping, gradient generation, gas–liquid exchange, and other processes. Integration of functional membranes in microfluidics, however, is nontrivial. Here, we report a simple approach for biofabricating freestanding, semi-permeable biopolymer membranes in microfluidics, initiated with intentionally trapped air bubbles caught within specifically designed polydimethylsiloxane (PDMS) apertures. Pressure-driven dissipation of air bubbles through the gas permeable PDMS facilitates local and quiescent contact of two oppositely charged polyelectrolyte polysaccharides forming a layered or sandwiched membrane. This polyelectrolyte complex membrane (PECM) is permeable to ions including hydroxyl ions, which further facilitates layer-by-layer assembly of membrane stratum. Assembled membranes that bridge the 40-μm apertures are sufficiently strong to withstand >1 atmosphere hydrostatic pressure. Further, the semi-permeable membranes allow for programmed generation of small molecule gradients while preventing protein efflux. We envision the simplicity of fabrication, which requires no reagents or complicated valving, when coupled with the functional properties of the membrane polysaccharides, will find utility in cell and tissue studies including preclinical drug screening and toxicity analyses.